Conventionally, as the above-mentioned copper wire rod, one made of the tough-pitch copper is broadly used. However, the tough-pitch copper cannot be used in such a case where the magnet wire is used by welding since the tough-pitch copper includes oxygen (O) at 0.02-0.05 mass % causing the hydrogen embrittlement. Therefore, in the use where welding is performed, the copper wire rod, which is made of copper with a low oxygen content such as the oxygen-free copper or the like whose oxygen content is 10 ppm or less by mass ppm, is used.
The above-mentioned copper wire rod is produced by dip forming or extruding. In the dip forming, the copper wire rod is obtained by continuously solidifying molten copper around a copper seed wire to obtain a rod-shaped copper material; and rolling. In the extruding, the copper wire rod is obtained by subjecting the copper billet to extruding; and performing rolling or the like. However, production efficiency is poor in these production methods increasing production cost.
As copper wire rod production methods with low production cost, there is the method by continuous casting and rolling using the belt-wheel type continuous casting apparatus and the continuous rolling device as described in Japanese Unexamined Patent Application, First Publication No. 2007-50440 (A), for example. In this continuous casting and rolling method, the molten copper, which is melted in a large-sized melting furnace such as the shaft furnace, is converted to an ingot by cooling and solidifying; and the ingot is subjected to continuous withdrawing and rolling. In this method, mass-production is possible by using a large-scale facility.
However, when copper with low oxygen content is melted, hydrogen concentration in the molten copper increases and water vapor bubbles are formed. Then, holes are formed in the ingot due to difficulty of the above-mentioned formed bubbles to be released from the bath level since the mold is in revolving movement in the belt-wheel type continuous casting apparatus.
It is believed that the above-mentioned holes residing in the ingot are the main cause of the surface defects in the copper wire rod. The surface defects of the copper wire rod also causes surface defects of the drawn wire material when the copper wire rod is subjected to a drawing process to be a drawn wire material. When such a drawn wire material is used for the conductor of the magnet wire and an enamel coat (insulating film) is applied on the surface of the drawn wire material, moisture or oil residing in the surface defect of the drawn wire material is trapped by the enamel coat. In this case, it causes a problem refereed “blister defect” formation, in which a bubble is generated and swollen in the enamel coat during heating after drying.
In order to suppress the formation of the blister defect, the copper wire rod, which is produce by adding a P-compound to the molten copper in such a way that the phosphorous (P) content of the ingot is set to 1-10 ppm and adjusting the temperature of the molten copper to 1085° C.-1100° C., is disclosed in Japanese Patent (Granted) Publication No. 4593397 (B), for example.